The present invention relates to apparatus for recording and/or reproducing high-frequency information on magnetic tape, and more particularly to a mechanism for guiding magnetic tape and one or more magnetic heads in such apparatus.
It is already known to guide magnetic tape along the peripheral surfaces to two coaxial drums which define an annular gap for one or more magnetic heads mounted in the interior of the drum and serving to record or reproduce information on the tape. The diameters of the peripheral surfaces are identical or nearly identical and the portion of tape which engages such peripheral surfaces forms a helical loop (e.g., an alpha loop or an omega loop). One of both drums may be driven or head against rotation about their common axis. The magnetic head or heads are mounted on a carrier or bracket which relates about the common axis of and is located between the drums. The plane in which the carrier rotates the head or heads coincides with the central plane of the gap between the peripheral surfaces of the drums.
In recording-reproducing apparatus which employ video tape, the speed at which the tape moves during recording may but need not equal the tape speed during reproduction. Thus, it is often desired to accelerate the reproduction, to make slow-motion reproductions of recorded information or to reproduce still images of a particular subject or scene. This creates problems in the aforedescribed apparatus wherein the tape portions which is trained around the peripheral surfaces of the drums forms a helix. As is known, such guidance of the tape is desirable because it allows for relatively high signal density in each of the scan lines which is particularly desirable in connection with recording of television signals, i.e., an entire image can be recorded on a single relatively long track. The track makes an acute angle with the marginal portions of the tape. The inclination of the track is determined primarily by two factors, namely by inclination of the tape with respect to the common axis of the drums and by speed of lengthwise movement of the tape. If the direction of movement of the tape about the drums is identical with the direction of rotation of the heads, the inclination of the track with respect to the marginal portions of the tape increases with increasing speed of the tape. On the other hand, the inclination of the track decreases if the tape is being advanced counter to the direction of rotation of the heads and the speed of the tape increases.
If the speed of reproduction in a video system is less than the speed during recording, i.e., if the reproduction is to furnish a slow-motion or still-image effect, portions of the heads leave the adjacent tracks with the result that the quality of a substantial portion of the reproduced image is less than satisfactory, i.e., the reproduction takes place with a much less satisfactory signal to noise ratio.
Certain presently known helical scan apparatus are equipped with complex and expensive electronic systems which are designed to compensate for or suppress the just described phenomena. It is also known to employ a mechanical compensating system which is designed to change the path of tape with respect to the drums, i.e., to change the lead of the helical loop which engages the drums. Such proposal is not satisfactory because the tape undergoes undesirable deformation and also because the change in lead is not reproducible, with a requisite degree of accuracy owing to friction with the drums.